1. Field of the Invention
The present invention relates to a traction control system for automotive vehicles, and specifically to a system which is capable of adjusting a driving force (a driving torque) delivered to each of drive road wheels by engine-power adjustment such as temporarily stopping fuel supply to the engine or decreasing the fuel-supply amount to the engine.
2. Description of the Prior Art
In case that the friction coefficient of the road surface is lower than a value that the driver imagines when accelerating, drive wheels tend to slip unintendedly, and thus adequate acceleration performance and high driving stability cannot be obtained. To avoid this, in modern vehicles a traction control system is often provided to properly adjust traction acting between the tire installed on the drive wheel and the road surface, depending on the fiction coefficient .mu. of the road surface. In order to suppress undesired slip (often called acceleration slip) of drive wheels, recently there have been proposed and developed various traction control systems. As is generally known, there are several controlled objects (or several controlled variables) with regard to which a traction control can be performed. In one, a braking-force adjustment type traction control system uses a wheel-brake cylinder pressure controlling actuator through which a driving force applied to drive wheels is properly reduced by actively or forcibly increasing the wheel-brake cylinder pressure. Such a braking-force adjustment type traction control system is superior in terms of a responsiveness of the driving-force control. However the braking-force adjustment type traction control system is inferior to others, in terms of the durability of brake parts i.e., friction elements such as brake pads, brake shoes or the like, because the wheel-brake cylinder is actuated every occurrences of so-called acceleration-slip. In another, a throttle-control type traction control system uses a throttle-opening control actuator through which the throttle opening of the throttle valve provided in the induction system is properly adjusted to decreasingly compensate the output power of the engine directly. However, in such a throttle-control type traction control system, even if the throttle is fully closed, a braking force (called a back torque) applied to the drive wheels due to engine braking can be increasingly adjusted only to a level equivalent to the engine idle speed, and thus the driving force applied to the drive wheels cannot be satisfactorily and rapidly reduced. Therefore such a throttle-control type traction control system is inferior to others from the viewpoint of a responsiveness of the traction control. In more later models with an engine which is designed to electronically control combustion (burning condition of air-fuel mixture in the engine cylinder), fuel-supply itself would be properly controlled so that the fuel-supply is temporarily stopped or decreased in designated cylinders of all engine cylinders to provide a sufficient back torque rapidly. Such a traction control system will be hereinbelow referred to as a "fuel-supply control type traction control system". For instance, in case of an engine which has a plurality of cylinders and in which the injections to the respective engine cylinders are electronically controlled or timed independently of each other, a calculated slip ratio is firstly derived from a target drive-wheel speed as the difference between the actual drive-wheel speed and the target drive-wheel speed, and the number of so-called fuel-cut cylinders is determined depending on the calculated slip ratio (corresponding to the desired decrement in driving force to be applied to the drive wheels) and simultaneously the cylinder number of at least one particular engine cylinder that be subjected to fuel-cut control action is determined. In this manner, the fuel-supply to the particular engine cylinders is stopped or cut temporarily to produce an adequate back torque timely and consequently to rapidly suppress or prevent acceleration-slip at the drive wheels. In such a fuel-supply control type traction control system, the previously-noted target drive-wheel speed is generally set at a value equivalent to a driven-wheel speed during constant-speed driving at a speed higher than a predetermined value, and set at or fixed at a preset value when the vehicle speed is lower than the predetermined value, for example when the vehicle begins to run. In vehicles with the previously-noted fuel-supply control type traction control system, in the event that the target drive-wheel speed is adjusted toward the driven-wheel speed (corresponding to a front-wheel speed in case of rear-wheel-drive vehicles) during the constant-speed driving at a speed above the predetermined value and as a result the drive-wheel speed becomes identical to the driven-wheel speed by way of forcible fuel-cut control action or decreasing control action in fuel-supply, there is no risk of stalling the engine, because the engine runs at comparatively high engine revolution speeds with great rotational inertia of rotating parts of the vehicle. On the other hand, in the event that the target drive-wheel speed is fixed at a preset value when the vehicle starts to run, the smaller the preset value, the greater the slip amount (or the slip velocity) calculated as the difference between the actual drive-wheel speed and the target drive-wheel speed, thus increasing the number of fuel-cut engine cylinders and consequently increasing an estimated value of the so-called back torque. When starting, a comparatively small preset value of the target drive-wheel speed is effective to rapidly and adequately suppress or prevent acceleration-slip at the drive wheels. But, if the preset value of the target drive-wheel speed is set at or fixed at an excessively small value, when the drive-wheel speed is adjusted toward and reaches the excessively small preset value with a high response, there is a possibility of engine-stall, since the engine revolution speed may drop down to a value remarkably less than the engine idle speed. Therefore, the previously-discussed preset value (the fixed value) of the target drive-wheel speed is conventionally set at a predetermined value above an idle speed at which the engine is warm and the engine runs without load with the accelerator pedal released. During the warm-engine idling, the operating temperature of the engine has been maintained within a steady state and the temperature of engine coolant has risen adequately. In contrast, when the engine is cold and the engine runs without load with the accelerator pedal released, the viscosity coefficient of lubricant (i.e., engine oil supplied to moving engine parts) is high and thus there is a great friction loss (power loss from friction) owing to a high viscous resistance. During such cold-engine idling in which the engine does not yet reach the operating temperature, it is necessary to adjust the idle speed toward a greater value to avoid undesired engine stall. The above-mentioned vehicle with an engine which is designed to electronically control a combusting condition of each of the engine cylinders, of course maintains the idle speed at a desired value by properly adjusting an opening of the idle valve or an amount of fuel fed through the injection system depending on the coolant temperature and thus by properly decreasing the air-fuel ratio so as to compensate the air-fuel mixture richer. As set out above, in conventional engines with an electronic combustion control system, the engine idle speed is usually preselected as a minimum engine revolution speed (or a minimum internal-combustion-engine revolution speed), based on the coolant temperature. Thus, in the case that the conventional fuel-supply control type traction control system comes into operation owing to wheel-slip which may take place at the drive wheels when starting with the cold engine, the drive-wheel speed is firstly adjusted toward the target drive-wheel speed (i.e., the preset value fixed at a specified value equal to or somewhat greater than the warm-engine idle speed) of the target drive-wheel speed. During the instantaneous adjustment of the drive-wheel speed, the engine speed tends to become less than the minimum engine revolution speed, since the warm-engine idle speed is less than the desired idle speed which is determined on the basis of the engine coolant temperature during cold-engine idling, and therefore there is a possibility of engine-stall when starting with the cold engine. To avoid engine-stall when starting with the cold engine, in the electronically-controlled internal combustion engine with the fuel-supply control type traction control system, in the event that the engine speed becomes less than the minimum engine revolution speed based on the coolant temperature, the engine control system operates to cancel the fuel-cut requirement to rapidly recover combustion in the respective engine cylinders. Rapid recovery from the fuel-cut state to the combusting state produces a rapid rise in the drive-wheel speed, and as a result the traction control system may often decide to initiate the fuel-cut control action irrespective of decrease in the slip ratio of the drive wheels. Thereafter, the engine control system itself may often operate to cancel again the fuel-cut requirement for the purpose of recovering to the combusting state. In this manner, when the fuel-cut control action and the recovery to the combusting state (the fuel-delivery to all of the engine cylinders through the injectors) are repeatedly executed owing to positive and negative fluctuations in the drive-wheel speed with respect to a desired value (the target drive-wheel speed fixed at the specified value), it is not easy to converge the drive-wheel speed on the desired value. Such undesirable hunting (fluctuations or oscillation in the drive-wheel speed) may result in unstable behavior of the vehicle particularly during traction control when starting with the cold engine.